This invention relates to recovering heat from fuel cell exhaust.
Fuel cells generate electrical energy by reacting two fuel gas streams with each other. One of the gases is referred to as an anode gas while the other is referred as a cathode gas. For example, certain fuel cells use a stream of gas that is rich in hydrogen as the anode gas and an air stream as the cathode gas. When the fuel cell is in use, the hydrogen in the anode gas reacts with oxygen in the cathode gas to generate the electrical energy. The reaction produces exhaust gases, which may include un-reacted fuel gases, impurities contained within the fuel gas streams, and chemical products of the reactions in the fuel cell. The reaction in the fuel cell also generates heat, thereby elevating the temperature of the exhaust gases.
Fuel cells are normally part of a system, known as a fuel cell system that typically includes a fuel processor (also referred to as a xe2x80x9creformerxe2x80x9d) for generating one of the fuel gases. For example, the fuel cell system of the example above includes a reformer that reacts a hydrocarbon, such as methane, with water to produce the hydrogen rich stream. Certain fuel cell systems also include an anode tail gas oxidizer unit (ATO) where the exhaust gases from the fuel cell are, for example, reacted with oxygen to eliminate environmentally unfriendly chemicals from the exhaust. The anode tail gas oxidizer unit generates more heat, further elevating the temperature of the exhaust gases.
In general, one aspect of the invention relates to a fuel cell system that includes a fuel cell stack, which during operation generates electrical energy by reacting two streams of reactant gases. The fuel cell stack also produces a fuel cell exhaust stream. An oxidizer unit is positioned to receive the fuel cell exhaust stream and oxidize at least a part of the fuel cell exhaust stream during operation, to produce an oxidizer exhaust stream. A heat recovery system receives the oxidizer exhaust stream and transfers at least some heat from the oxidizer exhaust stream to a stream of water to generate a heated stream of water. The water is used to humidify a fuel inlet stream for a fuel processor. In some embodiments, a temperature sensor is positioned to sense the temperature of the heated stream of water and a control system maintains the heated stream of water at a target temperature based on the sensed temperature by controlling the amount of the heat from the oxidizer exhaust stream that is transferred to the stream of water.
Embodiments of the invention may include one or more of the following features. In one type of embodiment of the fuel cell system, a cooling system is used to maintain the heated stream of water at the target temperature. During operation, the heat recovery system transfers some of the heat from the oxidizer exhaust stream to the cooling system thereby reducing the amount of heat that is transferred to the water stream. The control system controls the amount of heat that is transferred to cooling system based on the temperature of the heated stream of water to maintain the temperature of the heated water stream at the target temperature. The cooling system includes a coolant that, during operation, is made to flow through the heat recovery system to extract heat from the heat recovery system. The coolant also flows through a cooler and the cooler extracts the heat from the coolant. A pump drives the coolant through the heat recovery system and the coolant, thereby causing the coolant to extract the heat from the oxidizer exhaust stream based on the temperature of the heated stream of water. The control system further includes a controller that is programmed to generate a control signal based on the temperature of the heated stream of water. The pump drives the coolant based on the control signal to maintain the heated input at the target temperature.
In certain embodiments, the heat recovery system includes a first heat recovery device that transfers heat from the oxidizer exhaust stream to the stream of water to produce an hot water stream at a temperature above the target temperature and a second heat recovery device that transfers heat from the hot stream of water to the coolant to produce the heated stream of water at the target temperature.
In other embodiments, the heat recovery system includes a cavity positioned to receive the oxidizer exhaust stream, a heat exchange tube positioned in the cavity to extract heat from the oxidizer exhaust stream, an inner conductive tube positioned within the heat exchange tube to receive a first fluid, and at least one vane connecting the inner conductive tube to the heat exchange tube to concentrically position the inner conductive tube within the heat exchange tube. The vane conducts heat from the exchange tube to the inner conductive tube and the first fluid. The inner conductive tube and the exchange tube define an annular channel for receiving a second fluid and the heat exchange tube conducts heat from the oxidizer exhaust stream to the second fluid. One of the first fluid and the second fluid is the stream of water, while the other is the coolant.
In another type of embodiment of the fuel cell system, the temperature of the heated stream of water is maintained at the target temperature by controlling the amount of heated exhaust that is directed to the heat recovery system. A conduit receives the oxidizer exhaust stream from the oxidizer unit. The conduit has a branch that is connected to the heat recovery system and the control system includes a valve that is positioned within the conduit to control an amount of the oxidizer exhaust stream that is directed to the heat recovery system through the branch in response to the temperature of the heated input. Thus, the fuel cell system controls the amount of heat transferred to the stream of water.
In certain embodiments, the temperature sensor may be a thermocouple that generates a sensor signal corresponding to the temperature of the heated input. The control system includes a microprocessor that is programmed to generate a control signal based on the sensor signal and a motor, which deflects the valve in response to the control signal, thereby controlling the valve to determine the amount of heated exhaust that is directed to the heat recovery device.
In other embodiments, the size or shape of the temperature sensor varies with the temperature of the heated input, and the control system includes a linkage connecting the temperature sensor to the valve. The temperature sensor includes an element, such as a bimetal strip or a wax plug, whose size or shape varies as the temperature of the heated input changes. The linkage causes the valve to deflect as the size or shape of the temperature sensor varies, thereby causing the valve to control the amount of heated exhaust that is directed to the heat recovery device. The linkage and the temperature sensor are designed to control the amount of heated exhaust that is directed to the heat recovery device to maintain the heated input at the target temperature.
In a second general aspect of the invention, a method includes generating electrical energy by reacting a first stream of reactant gas (e.g., hydrogen or reformate) and a second stream of reactant gas (e.g., oxygen or air) in a fuel cell to produce a fuel cell exhaust stream (e.g., containing combustible gas such as anode exhaust), oxidizing at least a part of the fuel cell exhaust stream to produce an oxidizer exhaust stream, transferring at least some heat from the oxidizer exhaust stream to a stream of water to generate a heated stream of water, sensing the temperature of the heated stream of water, and maintaining the heated stream of water at a target temperature by controlling the amount of the heat from the oxidizer exhaust stream that is transferred to the stream of water based on the temperature of the heated stream of water.
Embodiments of the invention may also include one or more of the following features. Some of the heat from the oxidizer exhaust stream is transferred to a cooling system, thereby reducing the amount of heat that is transferred to the stream of water. By controlling the amount of heat from the oxidizer exhaust stream that is transferred to the cooling system based on the temperature of the heated stream of water, the temperature of the heated stream of water is maintained at the target temperature. The step of transferring the some of the heat from the oxidizer exhaust to a cooling system includes extracting heat from the heat recovery device using a coolant, and driving the coolant out of the heat recovery system, thereby causing the coolant to extract heat from the oxidizer exhaust stream. A control signal is generated based on the temperature of the heated stream of water and the coolant is driven out of the heat recovery system at a rate that is based on the generated control signal, thereby causing the coolant to extract heat from the oxidizer exhaust stream based on the temperature of the heated stream of water.
In certain embodiments heat is transferred from the oxidizer exhaust stream to the stream of water to produce an hot stream of water at a temperature above the target temperature and then some heat is transferred from the hot stream of water to the coolant to produce the heated stream of water at the target temperature.
In other embodiments, an amount of the oxidizer exhaust stream that is required to heat the stream of water to the target temperature is determined. The stream of water is heated with only the determined amount of oxidizer exhaust, thereby controlling the amount of heat transferred to the stream of water to maintain the temperature of the output stream at the target temperature. A sensor signal corresponding to the sensed temperature of the heated input is generated and a control signal is computed based on the generated sensor signal. The control signal is used to control a device that directs only the determined amount of oxidizer exhaust to a heat recovery system for heating the stream of water, thereby maintaining the heated input at the target temperature.
In another embodiment of the invention, a fuel cell system includes a fuel cell stack which during operation generates electrical energy by reacting a first stream of reactant gas and a second stream of reactant gas. The fuel cell stack also produces a fuel cell exhaust stream. The system includes an oxidizer unit that receives the fuel cell exhaust stream and oxidizes at least a part of the fuel cell exhaust stream during operation. A heat recovery system receives at least a part of an exhaust stream from the oxidizer and allows heat to transfer from the oxidizer exhaust stream to a water stream. The water stream is then sent to a fuel processor humidifier system and mixed with a hydrocarbon gas stream to form a humidified fuel processor fuel inlet stream. Since the vapor pressure of water varies with temperature, the temperature of this mixture may be controlled to maintain a desired steam to carbon ratio in the humidified fuel processor fuel inlet stream. In some embodiments, the oxidizer may have a housing that forms a structural portion of a humidifier system such that heat transfer is provided between the oxidizer unit and the humidifier system. As an example, the humidifier system may enclose at least a portion of the oxidizer unit, be adapted to receive a heated water stream and drip the heated water across an evaporation surface, and be further adapted to flow a hydrocarbon gas across the evaporation surface to form a humidified fuel processor fuel inlet stream. The evaporation surface can be, for example, ceramic, metal or plastic xe2x80x9cbarrelsxe2x80x9d or other shapes that provide surface area for the water to flow across. In some embodiments, the humidifier system can also include a coolant line that circulates a coolant to transfer heat between the coolant and the humidifier system, thus providing greater capacity and flexibility for thermal control of the system.
In yet another aspect of the invention, a method is provided wherein a fuel processor gaseous fuel stream is flowed across a heated water stream, and a temperature of the heated water stream is controlled to regulate an amount of water vapor that is passed into the fuel processor gaseous fuel stream. In some such embodiments, the heated water stream is dripped onto an evaporation surface, and the fuel processor gaseous fuel stream is flowed over the evaporation surface. Other embodiments may include regulating an amount of air fed to the oxidizer unit to maintain a desired temperature of a catalyst within the oxidizer unit, transferring heat from an exhaust of the oxidizer unit to a water stream to form the heated water stream, and transferring heat from the heated water stream to a coolant stream to maintain a desired temperature or the heated water stream. Other embodimetns are possible.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.